GARLAND, TX – A day after police killed two gunmen who tried to ambush a Garland, Texas, event featuring controversial cartoons of the Muslim Prophet Mohammed, details began to emerge about the shooters.

One suspect, identified as Elton Simpson by a federal law enforcement source, linked himself to ISIS in a tweet posted just before the attack.

He also was no stranger to federal investigators. In 2011, he was convicted of making a false statement involving international and domestic terrorism.

The other suspect, identified as Nadir Soofi by two federal law enforcement officials, was Simpson’s roommate in a Phoenix apartment.

He wasn’t well-known to federal law enforcement and was not on the FBI’s radar, one of the officials said. Investigators were combing through evidence retrieved from the shooters’ Arizona home to help piece together a timeline of how their plot came together, the official said.

Simpson and Soofi never made it inside the Curtis Culwell Center in Garland, where in addition to the cartoon contest, a right-wing Dutch politician who’s on an al Qaeda hit list was speaking Sunday evening.

A traffic officer working after-hours as security for the event and armed only with a service pistol killed both men, who were wearing body armor and carrying assault rifles, Garland Police Department spokesman Joe Harn told reporters Monday.

“He did what he was trained to do, and under the fire that he was put under, he did a very good job. And probably saved lives,” Harn said of the unidentified officer. “We think their strategy was to get into the event center, and they were not able to get past our perimeter that we had set up.”

An unarmed security officer working with the patrol officer was shot in the ankle, police said. None of the approximately 200 people attending the event was hurt.

Harn declined to call the incident a terror attack, saying the motive was still under investigation.

“We don’t know their intent, other than that they were willing to pull up and shoot police,” Harn said.

Links to ISIS?

Simpson apparently posted a tweet before the attack that read, in part, “May Allah accept us as mujahideen.”

The tweet from Simpson also said he and his fellow attacker had pledged loyalty to “Amirul Mu’mineen” (the leader of the faithful), which CNN terrorism analyst Paul Cruickshank said probably refers to ISIS leader Abu Bakr al Baghdadi.

After the shooting, an ISIS propagandist that Simpson had earlier asked his readers to follow tweeted, “Allahu Akbar!!!! 2 of our brothers just opened fire” at the Texas event.

“If there is no check on the freedom of your speech, then let your hearts be open to the freedom of our actions,” tweeted the propagandist, who was identified by two American groups that monitor jihadi websites as Junaid Hussain, a British ISIS fighter in Syria who goes by the name Abu Hussein al Britani.

In 2011, Simpson was convicted of making a false statement involving international and domestic terrorism and sentenced to three years of probation, court records show. Prosecutors said he told FBI agents that he had not discussed traveling to Somalia to engage in “violent jihad” when, in fact, he had, according to an indictment reviewed by CNN.

U.S. authorities are investigating whether Sunday’s shooting has any link to international terrorism. Simpson’s tweet could indicate the attack was inspired by ISIS, but not orchestrated by the group, sources said.

Similarities to attacks in Denmark, France

The incident bears similarities to attacks this year on events in France and Denmark featuring images of Mohammed, which some Muslims believe is blasphemous.

In January, gunmen attacked the offices of Charlie Hebdo, a French satirical magazine that has a controversial history of depicting Mohammed, and killed 12 people.

The next month, a gunman attacked a free speech forum in Copenhagen, Denmark, featuring cartoonist Lars Vilks, who infuriated al Qaeda with his depictions of Mohammed.

In the United States, cartoonist Molly Norris is still in hiding, four years after she depicted the likeness of Mohammed on several items and was deemed a “prime target” for execution by Islamic extremists.

The Sunday night event in Garland invited cartoonists to send in caricatures of Mohammed. It was organized by the American Freedom Defense Initiative — considered an anti-Muslim group by the Southern Poverty Law Center, which tracks hate groups.

Organizers said they received more than 350 submissions for the event.

The winning entry won $12,500. The black and white drawing shows a cartoonist’s hands sketching a sword-wielding Mohammed, who is shouting, “You can’t draw me!”

A speech bubble coming from the hands depicts the cartoonist’s response: “That’s why I draw you.”

“The Islamic jihadis are determined to suppress our freedom of speech violently.” Pamela Geller, president of the American Freedom Defense Initiative, told CNN. “They struck in Paris and Copenhagen recently, and now in Texas.”

Responding Monday to criticisms of her group as anti-Islamic, she said, “There is a problem in Islam, as illustrated last night, and anyone that addresses it gets attacked in this same way.”

Exchange lasted seconds

According to police, the men drove into the parking lot of the Curtis Culwell Center about 6:50 p.m. (7:50 ET), just as the “Muhammad Art Exhibit and Cartoon Contest” inside was ending.

The gunmen encountered a patrol car blocking the entrance, part of $10,000 in extra security paid for by organizers of the event, said Harn, the Garland Police Department spokesman. The American Freedom Defense Initiative said it had spent more than $30,000 on security.

As soon as the officers stepped out of the car to check the vehicle’s occupants, the two men left their vehicle and began firing, Harn said.

The unarmed security guard, Bruce Joiner, was shot in the ankle. He was later treated at a hospital and released, police said.

A SWAT reaction team raced to the parking lot to back up the patrol officer, and police inside the center shepherded audience members to a more secure location inside the center’s auditorium.

“There was an incident outside,” an officer told audience members. “Two suspects have been shot. Possibly have explosives on ’em, OK? I just need everybody to remain calm, become orderly, and we’re going to take you into the auditorium a little further away from the front of this building. All right?”

Someone asked, “Were the suspects Muslim?”

“I have no idea right now,” he responded.

It turned out the suspects did not have explosives, Harn said.

Heavy security for event

The American Freedom Defense Initiative said it specifically picked the venue, a school district-owned facility, because it hosted an event denouncing Islamophobia in January.

That event, “Stand with the Prophet,” was meant to counter Islamophobia after the Charlie Hebdo attack. It drew several hundred attendees and about 200 protesters, and went off without incident.

Culwell Center Director John Wilborn told The Dallas Morning News that the venue has yet to turn down an event because of content. The center is owned by the school district and rented out for sporting events, concerts and other gatherings.

Security was tight. The district brought in extra officers, and the group itself hired several more. Only those who purchased tickets ahead of time were admitted. They had to go through metal detectors.

“We were prepared for something like this,” Harn said.

Shortly after the Sunday night shooting, a prominent Muslim leader in Dallas tweeted about it.

“The community stayed away from event,” Imam Zia Sheikh wrote. “Seems like a lone wolf type of attack. Just what we didn’t want.”

‘A gentle person’

Members of the mosque Simpson once regularly attended, the Islamic Community Center of Phoenix, are in shock about what happened, said its president, Usama Shami.

Simpson was a regular worshiper at the mosque until around 2010 or 2011, about the time the FBI arrested him on the false statement charges.

During that time, he offered no signal that he held radical views, Shami said.

“He was a gentle person,” Shami said of Simpson. “He always had a good attitude, a good demeanor.”

‘Freedom of speech is under violent assault’

Wilders, the Dutch politician who was the keynote speaker at the Garland event, is controversial for his anti-Islam views. He was placed on the al Qaeda hit list for his film “Fitna.”

The film, which Wilders released online in March 2008 to international outcry, features disturbing images of terrorist acts superimposed over verses from the Quran in an apparent attempt to paint Islam as a threat to Western society.

In 2011, Wilders was cleared of charges of inciting discrimination and hatred with the movie.

“The day we give away humor and freedom of speech is the day that we cease to exist as a free and independent people,” he told the attendees at the Garland event Sunday night.

Likewise, the American Freedom Defense Initiative is known for its anti-Muslim stance.

Geller, its president, is “the anti-Muslim movement’s most visible and flamboyant figurehead,” the SPLC says.

“Who designated the SPLC as a legitimate authority? They are a radical leftist group who targets patriots, vets and even GOP presidential candidates,” she told CNN. “They have never named a jihadi group as a hate group.”

A conservative blogger, she first gained national attention with her group, “Stop the Islamization of America,” and its vocal opposition to an Islamic community center planned near the site of New York’s ground zero, where the twin towers of the World Trade Center were destroyed by Islamist hijackers on September 11, 2001.

She said Sunday night that she wasn’t expecting such an attack but wasn’t surprised that it happened.

“This incident shows how much needed our event really was. The freedom of speech is under violent assault here in our nation. The question now before us: Will we stand and defend it or bow to violence, thuggery and savagery?”

She said she plans on holding similar events.

“I will not abridge my freedoms so as not to offend savages,” she said.